468 CHEMICAL ENGINEERING
tance in series with the moving coil may be decreased. Because of the very small electromotive forces developed by thermocouples and the necessity of using a comparatively high resistance in series with the galvanometer coil to minimize the effect of a varying resistance, the torque which can be produced on the moving coil is small. The pointer cannot be used to trace a legible record directly since the friction between the pointer and paper would entirely alter the readings. One common method for obtaining the record is illustrated in Fig. 31. The paper is unwound from its roll, at uniform speed by clockwork. An inked ribbon lies below the paper and above a metal plate. At periodic intervals the chopper bar B falls pressing a stylus on the end of the galvanometer boom or needle N into contact with the paper and against the ribbon and metal plate underneath. This makes a small dot on the under side of the translucent paper which shows through from above as illustrated. The paper is ruled with the proper time and temperature coordinates and the row of dots obtained by continuous operation constitutes the required temperature-time curve. In* general the dots may be made to appear at intervals of 10 to 60 seconds, depending on the natural period of vibration of the moving coil. The above principle is employed in many instruments of American make. The chopper bar may be operated by an electric motor, clockwork, or electromagnet, and the design must be such that the galvanometer pointer swings clear of the bar between intervals of depression, and such that the depression of the bar against the boom or pointer in no way damages the coil mounting. These conditions may be met in suspension, pivot, and combination pivot-suspension systems.
Industrial Types of Recorders.—The Circular-chart single-point recorders are made by Cleveland Instrument Co., the Bristol Co., Brown Instrument Co., and Hoskins Manufacturing Co.
Roll chart recorders are made by the Brown Instrument Co., Bristol Co., Charles Engelhard, Hoskins Manufacturing Co., S. H. Stupakoff, Taylor Instrument Cos., Thwing Instrument Cos., and Wilson-Maeulen Co. The potentiometer recorder of the Leeds & Northrup Co., which has a roll chart, is described in a later section.
General advantages of the roll chart are legibility (coordinates are parallel and may be made rectangular), width of scale, adaptability to rapidly changing temperatures, and the recording of more than one record on a single chart. With a paper speed of 1 in. per hour a 20-yd. roll will last 1 month. Rolls are generally supplied in lengths of 20 yds. or longer.
This type of recorder is easily made multiple recording for several couples by use of a commutating device for automatically switching successively into the circuit the various couples. The number of records which can be thus made on a single chart without confusion is usually four to six although occasionally as many as 12 records are employed.
Thermocouple Recorder Operating on the Potentiometric Principle.—The principle of operation of the potentiometer circuit and the various arrangements as it is applied in pyrometry has been described (see Fig. 6). The three steps in the operation, (1) adjusting the current in the battery circuit, (2) connecting the thermocouple and (3) moving the slide-wire contact until the galvanometer deflection is zero, are done automatically in the multiple-point potentiometer recorder, made by the Leeds & Northrup Co. In the curve-drawing recorder, which is a single-point instrument-made by the same company, the battery current is adjusted by hand.
The potentiometer recorder consists essentially of a potentiometer with a mechanical movement for automatically changing the slide-wire contact and moving the pen or print wheel across the paper; also the parts necessary to move the paper